1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, an LCD device using a phase-difference film to realize a wide viewing angle.
2. Discussion of the Related Art
Among various ultra-thin flat type display devices having a thickness of several centimeters, the liquid crystal display (LCD) device is widely used for notebook computers, monitors, aircrafts, etc. Generally, an LCD device includes lower and upper substrates facing each other, and a liquid crystal layer disposed between the lower and upper substrates. As a voltage is applied to the LCD device, the light transmittance of the LCD device is controlled according to an alignment of the liquid crystal layer, thereby displaying images.
Hereinafter, a related art LCD device will be explained with reference to FIGS. 1A and 1B. FIGS. 1A and 1B are cross sectional views of a related art twisted nematic (TN) mode LCD device, wherein FIG. 1A illustrates the TN mode LCD device at the time of no voltage application, and FIG. 1B illustrates the TN mode LCD device at the time of voltage application.
The LCD device includes a first substrate 1, a second substrate 3, and a liquid crystal layer 5 formed between the first and second substrates 1 and 3. Then, a first alignment layer of a first direction is coated on the first substrate 1, and a second alignment layer of a second direction is coated on the second substrate 3, wherein the first direction of the first alignment layer is perpendicular to the second direction of the second alignment layer. Next, a first polarizer 7 having a transmission axis in the same direction as the first alignment layer is formed on an outer surface of the first substrate 1. Also, a second polarizer 9 having a transmission axis in the same direction as the second alignment layer is formed on an outer surface of the second substrate 3.
In the case of FIG. 1A, when the voltage is not applied to the LCD device, liquid crystal molecules included in the liquid crystal layer 5 are twisted at 90° between the first and second alignment layers. In this case, as an unpolarized light 10 is incident on the second polarizer 9, the unpolarized light is polarized to a linearly polarized light so that the linearly polarized light is transmitted through the liquid crystal layer 5. Like the twisted liquid crystal molecules, the polarization of the light is twisted by 90° so that light is transmitted through the first polarizer 7 and the image is displayed in a white state.
In the case of FIG. 1B, when the voltage is applied to the LCD device, the liquid crystal molecules included in the liquid crystal layer 5 are aligned in the vertical direction between the first and second substrates 1 and 3 due to the electric field generated. At this time, as an unpolarized light 10 is incident on the second polarizer 9, the unpolarized light is polarized to a linearly polarized light so that the linearly polarized light is transmitted through the liquid crystal layer 5. In this case, as the polarization direction of light is not twisted, the light is blocked by the first polarizer 7, thereby displaying a black state.
According to the driving method of FIGS. 1A and 1B, the TN mode LCD device displays the images. However, the TN mode LCD device has a disadvantage of a narrow viewing angle.
FIGS. 2A to 2C are perspective views for explaining a problem of the TN mode LCD device, wherein FIG. 2A illustrates a white display state at the time of no voltage application, FIG. 2B illustrates a black display state of full-voltage application, and FIG. 2C illustrates an intermediate display state at the time of intermediate-level voltage application.
In FIG. 2A, when the voltage is not applied to the LCD device, the liquid crystal molecules 5 are twisted, and the incident light displays the white state in all directions. In FIG. 2B, when the full-voltage is applied to the LCD device, the liquid crystal molecules 5 are aligned in the vertical direction by the electric field so that the incident light displays the black state. In FIG. 2C, when the intermediate-level voltage is applied, the liquid crystal molecules 5 are aligned in a tilt direction so that the display state is changed according to the direction of incident light. That is, in the case of the leftward-tilt light, which is incident from a lower right side to an upper left side, the polarization direction is not changed, thereby displaying the black state. Meanwhile, in the case of the rightward-tilt light, which is incident from a lower left side to an upper right side, the polarization direction is changed, thereby displaying the white state. Accordingly, the TN mode LCD device has the display state changed according to the light incidence direction so that the TN mode LCD device has the disadvantage of narrow viewing angle.
To overcome the problem of narrow viewing angle, various methods have been proposed. For example, an in-plane switching (IPS) mode using a transverse electric field parallel to the substrates, a vertical alignment (VA) mode using a vertical alignment layer, an electrically controlled birefringence (ECB) mode, etc. have been proposed. In addition, there is a multi-domain method which uses the mean value of alignment of liquid crystal molecules by dividing a domain, and a phase-difference compensation method which uses a phase-difference film to change a phase-difference according to a change of viewing angle. Among the above methods for overcoming the problem of narrow viewing angle, the phase-difference compensation method using a phase-difference film will be explained with reference to FIG. 3.
In the phase-difference compensation method of FIG. 3, a phase-difference film is formed above or below of a liquid crystal cell having liquid crystal molecules so that the same display state is obtained without regard to the light incidence direction.
The use of the phase-difference film to achieve a wide viewing angle is not limited to the TN mode. That is, the phase-difference film may be applied to the IPS mode or the ECB mode. To overcome the problem of the TN mode LCD device, the IPS mode or the ECB mode has been proposed. However, even in case of the IPS and ECB modes, it is difficult to completely overcome the problem of viewing angle. That is, the appropriate phase-difference film is applied based on the mode to improve the viewing angle.
The method using the phase-difference film is changed in design in consideration of the alignment state of liquid crystal based on the mode. Accordingly, the phase-difference film is formed of a material which is suitable for changing the design of phase-difference film. That is, a phase difference (Δnd) is designed by chaining a refractive anisotropy (Δn) of liquid crystal layer and a thickness (d) of liquid crystal layer, thereby optimizing the viewing angle according to the mode.
However, if fabricating the phase-difference film by the liquid crystal, the phase difference (Δnd) obtained by changing the refractive anisotropy (Δn) of liquid crystal layer and the thickness (d) of liquid crystal layer can compensate for the viewing angle. In the related art, there is no consideration to a tilt angle of liquid crystal layer. Thus, a light leakage may be caused due to the tilt angle of liquid crystal. If fabricating the film using the liquid crystal, the alignment layer must be provided for aligning the liquid crystal molecules. That is, the tilt angle is generated in the liquid crystal layer due to the alignment layer. When fabricating the phase-difference film using the liquid crystal according to the related art, the refractive anisotropy (Δn) of liquid crystal layer and the thickness (d) of liquid crystal layer are considered. That is, there is no consideration given to the tilt angle generated in the liquid crystal in fabricating of the phase-difference film such that the problem of light leakage is generated.